1. Field of the Invention
This invention relates to fluorescent lamp power supplies, and more particularly, to an inverter circuit for driving a cold cathode fluorescent lamp in a floating configuration.
2. Description of the Related Art
The use of fluorescent lamps continues to increase as systems requiring an efficient and broad-area source of visible light become essential for various consumer electronic devices. For example, the use of portable computers such as laptop and notebook computers is rapidly increasing. In portable computers, fluorescent lamps are used to back-light or side-light liquid crystal displays to improve the contrast or brightness of the display. Other examples of the use of fluorescent lamps includes illuminating automobile dashboards and commercial signage.
Fluorescent lamps are used in various applications due to their energy efficiency and their ability to diffuse light over a broad area compared to other lighting sources. The increased efficiency of fluorescent lamps becomes particularly important in battery-driven devices, where longer battery life translates to being able to use the device for a longer period of time without recharging the battery or having to find an alternate power source. The relative efficiency of fluorescent lamps notwithstanding, in portable equipment, such as a laptop computer, the back-light can account for as much as 40% of the total equipment power drain. In applications where portability is important, further advantage is gained where smaller and more lightweight battery packs may be used due to the energy efficiency of the device.
In many portable device applications, however, extended battery life is often limited by energy losses, such as those due to parasitic energy paths. For example, fluorescent lamps are traditionally driven by signals input to one end of the lamp, where one end of the lamp is coupled to a sinusoidal drive signal and the other end of the lamp is held at essentially ground potential. The parasitic energy loss is relatively high due to the high amplitude required to drive the lamp to fully illuminate it. This energy loss translates into decreased battery life or heavier batteries, or both.
In notebook computers, an inverter circuit is typically used to convert unregulated DC voltage to regulated AC current to provide power to drive, also referred to as illuminating, the fluorescent lamp. The inverter circuit is typically mounted on one of the sides of the display panel, thereby adding width to the panel assembly. In the past, the keyboard in a laptop computer was usually wider than the display, however, as display size increases beyond the size of the keyboard in more modern laptop computers, it is desirable to move the inverter circuit from the side of the display to another location to avoid increasing the width of the housing.
In view of the foregoing, it is therefore desirable to provide an inverter circuit for a cold cathode fluorescent lamp that minimizes energy loss.
It is also desirable to provide a display assembly for a portable devices that is lightweight and compact.